Connection of electrical contacts utilizing a combination laser and fiber optic push connect system

ABSTRACT

This disclosure describes a method for the solderless electrical connection of two contact elements by using a laser light beam attached to a fiber optic system which directs the light to the spot to be bonded. By using a fiber optic system the laser beam is optimally converted into thermal energy and bad connections due to underheating or destruction of the contacts due to overheating does not occur. The method and apparatus provides rapid, reproducible bonding even for the smallest of contact geometries. For example, the method of the invention results in solderless gold to gold compression bonding of conductive leads contained in a polymer flex circuit tape, such as a polyimide, without damaging the tape. A strong solderless gold to gold bond can be formed between the gold plated copper lead on the flex circuit tape and a gold plated pad on a semiconductor chip without the need for a window in the flex circuit and without any damage to the tape. In the application of the present invention to the bonding of conductive leads on a TAB circuit to the silicon substrate of an inkjet printhead the need for a window in the TAB circuits is eliminated. The elimination of the window results in elimination of the need for an encapsulation material to cover the conductive leads in the TAB circuit. This in turn results in die size reduction, or increased number of nozzles with the same die size, ease of assembly, higher yields, improved reliability, ease of nozzle serviceability, and overall material and manufacturing cost reduction.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application relates to the subject matter disclosed in thefollowing U.S. Patents and co-pending U.S. Applications:

[0002] U.S. Pat. No. 5,442,384, entitled “Integrated Nozzle Member andTAB Circuit for Inkjet Printhead;” and

[0003] U.S. Pat. No. 5,278,584, entitled “Ink Delivery System for anInkjet Printhead; ”

[0004] The above patent and co-pending applications are assigned to thepresent assignee and are incorporated herein by reference.

FIELD OF THE INVENTION

[0005] The present invention generally relates to the electricalconnection of two elements and, more particularly, to the solderlessconnection of two elements using an optical fiber that holds theelectrical elements in contact while directing a laser emission to thelocation to be bonded.

BACKGROUND OF THE INVENTION

[0006] Thermal inkjet print cartridges operate by rapidly heating asmall volume of ink to cause the ink to vaporize and be ejected throughone of a plurality of orifices so as to print a dot of ink on arecording medium, such as a sheet of paper. The properly sequencedejection of ink from each orifice causes characters or other images tobe printed upon the paper as the printhead is moved relative to thepaper.

[0007] An inkjet printhead generally includes: (1) ink channels tosupply ink from an ink reservoir to each vaporization chamber proximateto an orifice; (2) a metal orifice plate or nozzle member in which theorifices are formed in the required pattern; and (3) a silicon substratecontaining a series of thin film resistors, one resistor pervaporization chamber.

[0008] To print a single dot of ink, an electrical current from anexternal power supply is passed through a selected thin film resistor.The resistor is then heated, in turn superheating a thin layer of theadjacent ink within a vaporization chamber, causing explosivevaporization, and, consequently, causing a droplet of ink to be ejectedthrough an associated orifice onto the paper.

[0009] In U.S. application Ser. No. 07/862,668, filed Apr. 2, 1992,entitled “Integrated Nozzle Member and TAB Circuit for InkjetPrinthead,” a novel nozzle member for an inkjet print cartridge andmethod of forming the nozzle member are disclosed. This integratednozzle and tab circuit design is superior to the orifice plates forinkjet printheads formed of nickel and fabricated by lithographicelectroforming processes. A barrier layer includes vaporizationchambers, surrounding each orifice, and ink flow channels which providefluid communication between a ink reservoir and the vaporizationchambers. A flexible tape having conductive traces formed thereon hasformed in it nozzles or orifices by Excimer laser ablation. By providingthe orifices in the flexible circuit itself, the shortcomings ofconventional electroformed orifice plates are overcome. The resultingnozzle member having orifices and conductive traces may then havemounted on it a substrate containing heating elements associated witheach of the orifices. Additionally, the orifices may be formed alignedwith the conductive traces on the nozzle member so that alignment ofelectrodes on a substrate with respect to ends of the conductive tracesalso aligns the heating elements with the orifices. The leads at the endof the conductive traces formed on the back surface of the nozzle memberare then connected to the electrodes on the substrate and provideenergization signals for the heating elements. The above procedure isknown as Tape Automated Bonding (“TAB”) of an inkjet printhead assembly,or TAB Head Assembly, (hereinafter referred to as a “THA”)

[0010] An existing solution for connecting the conductive traces formedon the back surface of the nozzle member to the electrodes on thesubstrate for a THA requires a flexible TAB circuit with a window in theKapton tape. This window provides an opening for the bonder head, whichpermits direct contact of the thermode (single point or gang) with theTAB leads. Therefore, the attach process is performed without directcontact between the thermode and Kapton tape. A TAB bonder thermodecomes in direct contact with the flex circuit copper TAB leads throughthis window. The thermode provides the thermal compression forcerequired to connect the TAB conductive leads to the printhead substrateelectrode. Alternatively, ultrasonic method may be used to connect theTAB conductive leads to the printhead substrate electrode. This windowis then filled with an encapsulation material to minimize damage to theconductive leads, shorting, and current leakage. This encapsulationmaterial may flow into the nozzles and cause blockages. Therefore, theTab Head Assembly (“THA”) is designed in a manner that allows a 0.50 to0.75 mm gap between the edge of the encapsulation and the nozzles. Thisincreases the substrate size by 1 to 1.5 mm. The encapsulation materialalso creates an impression that is not desirable for serviceability, andcreates co-planarity, and reliability problems.

[0011] Accordingly, it would be advantageous to have a process thateliminates the need for a window in the TAB circuits. The elimination ofthe window results in elimination of the need for an encapsulationmaterial to cover the conductive leads in the TAB circuit. This in turnwould result in die size reduction, or increased number of nozzles withthe same die size, ease of assembly, higher yields, improvedreliability, ease of surface serviceability, and overall material andmanufacturing cost reduction.

SUMMARY OF THE INVENTION

[0012] The present invention provides a method for the solderlesselectrical connection of two contact elements by using a laser lightbeam attached to a fiber optic system which directs the light to thespot to be bonded. By using a fiber optic system the laser beam isoptimally converted into thermal energy and bad connections due tounderheating or destruction of the contacts due to overheating does notoccur. The method and apparatus provides rapid, reproducible bondingeven for the smallest of contact geometries. For example, the method ofthe invention results in solderless gold to gold compression bonding ofconductive leads contained in a polymer flex circuit tape, such as apolyimide, without damaging the tape. A strong solderless gold to goldbond can be formed between the gold plated copper lead on the flexcircuit tape and a gold plated pad on a semiconductor chip without theneed for a window in the flex circuit and without any damage to thetape.

[0013] In the application of the present invention to the bonding ofconductive leads on a TAB circuit to the silicon substrate of an inkjetprinthead the need for a window in the TAB circuits is eliminated. Theelimination of the window results in elimination of the need for anencapsulation material to cover the conductive leads in the TAB circuit.This in turn results in die size reduction, or increased number ofnozzles with the same die size, ease of assembly, higher yields,improved reliability, ease of nozzle serviceability, and overallmaterial and manufacturing cost reduction.

[0014] While the present invention will be described, for purposes ofillustration only, in conjunction with the bonding of conductive leadson a TAB circuit to the silicon substrate of an inkjet printhead, thepresent method and apparatus for the solderless electrical connection oftwo contact elements by using a laser light beam attached to a fiberoptic system is applicable to bonding any electrical members to eachother.

[0015] Other advantages will become apparent after reading thedisclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] The present invention can be further understood by reference tothe following description and attached drawings which illustrate thepreferred embodiment.

[0017] Other features and advantages will be apparent from the followingdetailed description of the preferred embodiment, taken in conjunctionwith the accompanying drawings, which illustrate, by way of example, theprinciples of the invention.

[0018]FIG. 1 is a perspective view of an inkjet print cartridgeaccording to one embodiment of the present invention.

[0019]FIG. 2 is a perspective view of the front surface of the TapeAutomated Bonding (TAB) printhead assembly (hereinafter “TAB headassembly”) removed from the print cartridge of FIG. 1.

[0020]FIG. 3 is a perspective view of an simplified schematic of theinkjet print cartridge of FIG. 1. for illustrative purposes.

[0021]FIG. 4 is a perspective view of the front surface of the TapeAutomated Bonding (TAB) printhead assembly (hereinafter “TAB headassembly”) removed from the print cartridge of FIG. 3.

[0022]FIG. 5 is a perspective view of the back surface of the TAB headassembly of FIG. 4 with a silicon substrate mounted thereon and theconductive leads attached to the substrate.

[0023]FIG. 6 is a side elevational view in cross-section taken alongline A-A in FIG. 5 illustrating the attachment of conductive leads toelectrodes on the silicon substrate.

[0024]FIG. 7 is a top perspective view of a substrate structurecontaining heater resistors, ink channels, and vaporization chambers,which is mounted on the back of the TAB head assembly of FIG. 4.

[0025]FIG. 8 illustrates one process which may be used to form thepreferred TAB head assembly.

[0026]FIG. 9 is a schematic diagram for a fiber push connect lasersystem as used in the present invention.

[0027]FIG. 10 shows in detail the flex circuit, the contact bond point,the TAB lead and die pad.

[0028]FIG. 11 shows the temperature profile of the flex circuit, TABlead, bond location and die pad during the bonding process with the FPClaser.

[0029]FIG. 12 shows the absorption property versus wavelength forvarious metals.

[0030]FIG. 13 ilustrates the optical transmission results for fivesamples of the Kapton tape sputtered with 2, 5, 10, 15, and 25 nm ofchromium.

[0031]FIG. 14 illustrates the temperature rise in flex circuits withTi/W seed layers.

[0032]FIG. 15 illustrates the temperature rise in flex circuits with achromium seed layer.

[0033]FIG. 16 illustrates temperature increase versus time in a 3-layertape with different thickness chromium seed layers.

[0034]FIG. 17 shows the results of a laser bonding experiment toevaluate the laser bondability of a flex circuit.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] While the present invention will be described, for purposes ofillustration only, in conjunction with the bonding of conductive leadson a TAB circuit to the silicon substrate of an inkjet printhead, thepresent method and apparatus for the solderless electrical connection oftwo contact elements by using a laser light beam attached to a fiberoptic system is applicable to bonding other types of electrical membersto each other.

[0036] Referring to FIG. 1, reference numeral 10 generally indicates aninkjet print cartridge incorporating a printhead according to oneembodiment of the present invention simplified for illustrativepurposes. The inkjet print cartridge 10 includes an ink reservoir 12 anda printhead 14, where the printhead 14 is formed using Tape AutomatedBonding (TAB). The printhead 14 (hereinafter “TAB head assembly 14”)includes a nozzle member 16 comprising two parallel columns of offsetholes or orifices 17 formed in a flexible polymer flexible circuit 18by, for example, laser ablation.

[0037] A back surface of the flexible circuit 18 includes conductivetraces 36 formed thereon using a conventional photolithographic etchingand/or plating process. These conductive traces 36 are terminated bylarge contact pads 20 designed to interconnect with a printer. The printcartridge 10 is designed to be installed in a printer so that thecontact pads 20, on the front surface of the flexible circuit 18,contact printer electrodes providing externally generated energizationsignals to the printhead. Bonding areas 22 and 24 in the flexiblecircuit 18 are where the bonding of the conductive traces 36 toelectrodes on a silicon substrate containing heater resistors occurs.

[0038] In the print cartridge 10 of FIG. 1, the flexible circuit 18 isbent over the back edge of the print cartridge “snout” and extendsapproximately one half the length of the back wall 25 of the snout. Thisflap portion of the flexible circuit 18 is needed for the routing ofconductive traces 36 which are connected to the substrate electrodesthrough the far end window 22. The contact pads 20 are located on theflexible circuit 18 which is secured to this wall and the conductivetraces 36 are routed over the bend and are connected to the substrateelectrodes through the windows 22, 24 in the flexible circuit 18.

[0039]FIG. 2 shows a front view of the TAB head assembly 14 of FIG. 1removed from the print cartridge 10 and prior to windows 22 and 24 inthe TAB head assembly 14 being filled with an encapsulant. TAB headassembly 14 has affixed to the back of the flexible circuit 18 a siliconsubstrate 28 (not shown) containing a plurality of individuallyenergizable thin film resistors. Each resistor is located generallybehind a single orifice 17 and acts as an ohmic heater when selectivelyenergized by one or more pulses applied sequentially or simultaneouslyto one or more of the contact pads 20.

[0040] The orifices 17 and conductive traces 36 may be of any size,test, and pattern, and the various figures are designed to simply andclearly show the features of the invention. The relative dimensions ofthe various features have been greatly adjusted for the sake of clarity.

[0041] The orifice 17 pattern on the flexible circuit 18 shown in FIG. 2may be formed by a masking process in combination with a laser or otheretching means in a step-and-repeat process, which would be readilyunderstood by one of ordinary skilled in the art after reading thisdisclosure. FIG. 14, to be described in detail later, providesadditional details of this process. Further details regarding TAB headassembly 14 and flexible circuit 18 are provided below.

[0042]FIG. 3 is a perspective view of a simplified schematic of theinkjet print cartridge of FIG. 1 for illustrative purposes. FIG. 4 is aperspective view of the front surface of the Tape Automated Bonding(TAB) printhead assembly (hereinafter “TAB head assembly”) removed fromthe simplified schematic print cartridge of FIG. 3.

[0043]FIG. 5 shows the back surface of the TAB head assembly 14 of FIG.4 showing the silicon die or substrate 28 mounted to the back of theflexible circuit 18 and also showing one edge of the barrier layer 30formed on the substrate 28 containing ink channels and vaporizationchambers. FIG. 7 shows greater detail of this barrier layer 30 and willbe discussed later. Shown along the edge of the barrier layer 30 are theentrances to the ink channels 32 which receive ink from the inkreservoir 12. The conductive traces 36 formed on the back of theflexible circuit 18 terminate in contact pads 20 (shown in FIG. 4) onthe opposite side of the flexible circuit 18 at location 38. The bondingareas 22 and 24 locate where the conductive traces 36 and the substrateelectrodes 40 (shown in FIG. 6) are bonded by using a laser light beamattached to a fiber optic system which directs the light to the locationto be bonded in accordance with the present invention.

[0044]FIG. 6 shows a side view cross-section taken along line A-A inFIG. 5 illustrating the connection of the ends of the conductive traces36 to the electrodes 40 formed on the substrate 28. As seen in FIG. 6, aportion 42 of the barrier layer 30 is used to insulate the ends of theconductive traces 36 from the substrate 28. Also shown in FIG. 6 is aside view of the flexible circuit 18, the barrier layer 30, the bondingareas 22 and 24, and the entrances of the various ink channels 32.Droplets of ink 46 are shown being ejected from orifice holes associatedwith each of the ink channels 32.

[0045]FIG. 7 is a front perspective view of the silicon substrate 28which is affixed to the back of the flexible circuit 18 in FIG. 5 toform the TAB head assembly 14. Silicon substrate 28 has formed on it,using conventional photolithographic techniques, two rows or columns ofthin film resistors 70, shown in FIG. 7 exposed through the vaporizationchambers 72 formed in the barrier layer 30.

[0046] In one embodiment, the substrate 28 is approximately one-halfinch long and contains 300 heater resistors 70, thus enabling aresolution of 600 dots per inch. Heater resistors 70 may instead be anyother type of ink ejection element, such as a piezoelectric pump-typeelement or any other conventional element. Thus, element 70 in all thevarious figures may be considered to be piezoelectric elements in analternative embodiment without affecting the operation of the printhead.Also formed on the substrate 28 are electrodes 74 for connection to theconductive traces 36 (shown by dashed lines) formed on the back of theflexible circuit 18.

[0047] A demultiplexer 78, shown by a dashed outline in FIG. 7, is alsoformed on the substrate 28 for demultiplexing the incoming multiplexedsignals applied to the electrodes 74 and distributing the signals to thevarious thin film resistors 70. The demultiplexer 78 enables the use ofmuch fewer electrodes 74 than thin film resistors 70. Having fewerelectrodes allows all connections to the substrate to be made from theshort end portions of the substrate, as shown in FIG. 4, so that theseconnections will not interfere with the ink flow around the long sidesof the substrate. The demultiplexer 78 may be any decoder for decodingencoded signals applied to the electrodes 74. The demultiplexer hasinput leads (not shown for simplicity) connected to the electrodes 74and has output leads (not shown) connected to the various resistors 70.The demultiplexer 78 circuity is discussed in further detail below.

[0048] Also formed on the surface of the substrate 28 using conventionalphotolithographic techniques is the barrier layer 30, which may be alayer of photoresist or some other polymer, in which is formed thevaporization chambers 72 and ink channels 80. A portion 42 of thebarrier layer 30 insulates the conductive traces 36 from the underlyingsubstrate 28, as previously discussed with respect to FIG. 4.

[0049] In order to adhesively affix the top surface of the barrier layer30 to the back surface of the flexible circuit 18 shown in FIG. 5, athin adhesive layer 84 (not shown), such as an uncured layer ofpoly-isoprene photoresist, is applied to the top surface of the barrierlayer 30. A separate adhesive layer may not be necessary if the top ofthe barrier layer 30 can be otherwise made adhesive. The resultingsubstrate structure is then positioned with respect to the back surfaceof the flexible circuit 18 so as to align the resistors 70 with theorifices formed in the flexible circuit 18. This alignment step alsoinherently aligns the electrodes 74 with the ends of the conductivetraces 36. The traces 36 are then bonded to the electrodes 74. Thisalignment and bonding process is described in more detail later withrespect to FIG. 8. The aligned and bonded substrate/flexible circuitstructure is then heated while applying pressure to cure the adhesivelayer 84 and firmly affix the substrate structure to the back surface ofthe flexible circuit 18.

[0050]FIG. 8 illustrates one method for forming the TAB head assembly14. The starting material is a Kapton or Upilex type polymer tape 104,although the tape 104 can be any suitable polymer film which isacceptable for use in the below-described procedure. Some such films maycomprise teflon, polyamide, polymethylmethacrylate, polycarbonate,polyester, polyamide polyethylene-terephthalate or mixtures thereof.

[0051] The tape 104 is typically provided in long strips on a reel 105.Sprocket holes 106 along the sides of the tape 104 are used toaccurately and securely transport the tape 104. Alternately, thesprocket holes 106 may be omitted and the tape may be transported withother types of fixtures.

[0052] In the preferred embodiment, the tape 104 is already providedwith conductive copper traces 36, such as shown in FIGS. 2, 4 and 5,formed thereon using conventional metal deposition and photolithographicprocesses. The particular pattern of conductive traces depends on themanner in which it is desired to distribute electrical signals to theelectrodes formed on silicon dies, which are subsequently mounted on thetape 104.

[0053] In the preferred process, the tape 104 is transported to a laserprocessing chamber and laser-ablated in a pattern defined by one or moremasks 108 using laser radiation 110, such as that generated by anExcimer laser 112. The masked laser radiation is designated by arrows114.

[0054] In a preferred embodiment, such masks 108 define all of theablated features for an extended area of the tape 104, for exampleencompassing multiple orifices in the case of an orifice pattern mask108, and multiple vaporization chambers in the case of a vaporizationchamber pattern mask 108.

[0055] The laser system for this process generally includes beamdelivery optics, alignment optics, a high precision and high speed maskshuttle system, and a processing chamber including a mechanism forhandling and positioning the tape 104. In the preferred embodiment, thelaser system uses a projection mask configuration wherein a precisionlens 115 interposed between the mask 108 and the tape 104 projects theExcimer laser light onto the tape 104 in the image of the patterndefined on the mask 108. The masked laser radiation exiting from lens115 is represented by arrows 116. Such a projection mask configurationis advantageous for high precision orifice dimensions, because the maskis physically remote from the nozzle member. After the step oflaser-ablation, the polymer tape 104 is stepped, and the process isrepeated.

[0056] A next step in the process is a cleaning step wherein the laserablated portion of the tape 104 is positioned under a cleaning station117. At the cleaning station 117, debris from the laser ablation isremoved according to standard industry practice.

[0057] The tape 104 is then stepped to the next station, which is anoptical alignment station 118 incorporated in a conventional automaticTAB bonder, such as an inner lead bonder commercially available fromShinkawa Corporation, Model No. ILT-75. The bonder is preprogrammed withan alignment (target) pattern on the nozzle member, created in the samemanner and/or step as used to created the orifices, and a target patternon the substrate, created in the same manner and/or step used to createthe resistors. In the preferred embodiment, the nozzle member materialis semi-transparent so that the target pattern on the substrate may beviewed through the nozzle member. The bonder then automaticallypositions the silicon dies 120 with respect to the nozzle members so asto align the two target patterns. Such an alignment feature exists inthe Shinkawa TAB bonder. This automatic alignment of the nozzle membertarget pattern with the substrate target pattern not only preciselyaligns the orifices with the resistors but also inherently aligns theelectrodes on the dies 120 with the ends of the conductive traces formedin the tape 104, since the traces and the orifices are aligned in thetape 104, and the substrate electrodes and the heating resistors arealigned on the substrate. Therefore, all patterns on the tape 104 and onthe silicon dies 120 will be aligned with respect to one another oncethe two target patterns are aligned.

[0058] Thus, the alignment of the silicon dies 120 with respect to thetape 104 is performed automatically using only commercially availableequipment. By integrating the conductive traces with the nozzle member,such an alignment feature is possible. Such integration not only reducesthe assembly cost of the printhead but reduces the printhead materialcost as well.

[0059] The automatic TAB bonder then uses a gang bonding method to bondthe conductive traces down onto the associated substrate electrodes.Higher bond temperatures are generally preferred to decrease the bondtime, but higher bond temperatures will soften the flex circuit andcause more deformation of the Kapton tape. It is extremely preferred tohave higher temperature at the contact point and lower temperature atthe Kapton tape layer. This optimum contact temperature profile may beachieved by utilizing a Fiber Push Connect (FPC) single point laserbonding process FPC in conjunction with a windowless TAB circuitprovides an ideal solution for a TAB head assembly for an inkjet printerprinthead.

[0060] A schematic for a FPC laser system 200 is illustrated in FIG. 9.This system consists of an Nd YANG or Diode laser 202, equipped with aglass (SiO2) optical fiber 204. The system guides the laser beam to thecontact or attach point 206 via the optical glass fiber 204. An optimumthermal coupling is achieved by pressing two parts together by means ofthe fiber 204 which creates a zero contact gap between the TAB lead 208and die pad 210 and thus improved thermal efficiency. FIG. 10 shows ingreater detail the flex circuit 18, the contact point 206, the TAB lead208 and die pad 210.

[0061] Referring to FIG. 9, a feedback temperature loop is achieved bymeans of an infrared detector 212 through the glass fiber. Thetemperature or absorption behavior response of the IR-radiationreflected by the contact elements 208, 210 at the contact point 206 isgathered. The outgoing laser beam 220 from the laser source 202 goesthrough a half-transmission mirror or beam splitter 214 and through afocussing lens 216 into the glass fiber optic 204. The reflected light218 from the fiber optic shown with dashed lines is reflected by thehalf mirror 21 and arrives via focussing lens 222 at an IR detector 212that is connected to a PC Controller 224. The graph shown on the monitor226 of PC controller 224 is meant to show that the PC Controller 224 canstore definite expected plots for the temperature variation of thebonding process with which the actual temperature variation can becompared. The PC Controller 224 is connected with the laser source 202so that the laser parameters can be controlled if necessary.

[0062] The reproducibility of a FPC laser bond depends both on a highdegree of thermal coupling between the two connectors 208, 210 and highabsorption of the laser energy by conductive leads 208, 210. To optimizethe bonding process, minimum absorption is desired in the Kapton tapeand maximum absorption is desired in the flex circuit 18 metal layer.Metals with higher absorption rate will transform a higher share of thelaser energy into heat. This will result in a shorter attach processwhich in turn will result in a higher quality bond.

[0063] The laser utilized is a YAG laser with a wavelength of 1064 nm.FIG. 12 illustrates the absorption property versus wavelength forseveral metals. As can be observed from FIG. 12, chromium and molybdenumhave the highest absorption characteristics at this wavelength. chromiumwas selected as the base metal due to the fact that most flex circuitmanufacturers are using chromium as the seed layer. The penetrationdepth of the laser into chromium is about 10 nm with a spot size of 5nm, thus requiring a minimum chromium thickness of 15 nm. The laser beamcreates a localized heated zone causing the metals (or solder material),to melt and create a bond between two joining surfaces withoutincreasing the temperature of the Kapton tape. However, any gap betweentwo mating metal parts will cause over heating of the metal surfaceexposed to the laser beam. This will cause deformation of the TAB leadswith no bond between metal surfaces. Also, an increased temperature inthe flex will cause damage to the flex circuit.

[0064]FIG. 11 illustrates a typical temperature profile of the flexcircuit 18 during bonding process with the FPC laser. As it can beobserved from FIG. 11, the temperature at the attach area 206 isconsiderably higher than the Kapton tape 18 temperature. This isachieved due to the high degree of the transparency of the Kapton tapeat different wavelengths.

[0065] The Kapton polyimide tape is transparent to the YAG laser beamand the laser beam passes through the 2 mil thick layer of polyimidewithout any absorption. Chromium is a conventional seed layer that isused extensively to provide an adhesion layer between the copper traceand Kapton polyimide in a two-layer flex circuit manufacturing process.A chromium layer with a minimum thickness of 10 nm (or 20 nm nominal) isrequired to provide a media which absorbs the laser energy. Thethickness of the chromium layer varies depending upon the flex circuitmanufacturer, with reported thicknesses between 2 and 30 nm. A typicalflex circuit manufacturing process utilizes a thin layer (20 nm) ofsputtered chromium as a seed (adhesion) layer between the copper tracesand Kapton polyimide.

[0066] Five samples of the Kapton tape were sputtered with 2, 5, 10, 15,and 25 nm of chromium, and optical transmission was measured for thesesamples. FIG. 13 illustrates the optical transmission results for thesesamples. It can be seen that optical transmission initially dropsrapidly with increased chromium thickness (from 65% for 2 nm ofchromium, to 12% for 15 nm of chromium), but optical transmissionchanges very slowly when chromium thickness increases from 15 to 25 nm.

[0067] Laser bonding process requires a fast temperature rise in theconductive trace to minimize the temperature rise in the Kapton andtherefore minimize damage to the Kapton tape. FIGS. 14 and 15 illustratetemperature rise in several flex circuits with different constructions.FIG. 14 illustrates temperature rise in flex circuits with thicker seedlayers. It is important to notice that flex circuits with 10 nm or lessof Ti/W did not reach the temperature that is required for gold/goldbonding, but the flex circuit with 20 nm of Ti/W did reach the bondingtemperature. Also, it should be noted that the rise time in the flexcircuit with thicker Ti/W is faster, minimizing the potential of damagedue to high localized temperatures in the Kapton tape.

[0068] The temperature (IR-Signal) fluctuation in the flex circuit with20 nm of Ti/W is indicative of the fact that this flex circuit reachedthe maximum preset temperature required for gold/gold bonding and thenthe laser feed-back loop temporarily dropped the laser energy so thatincrease in the TAB bond temperature did not damage the Kapton tape. Assoon as the temperature of the Kapton tape dropped (by a preset amount),the laser energy automatically increased to fill power to increase theTAB lead temperature, and created a reliable gold/gold bond.

[0069]FIG. 15 illustrates similar results for different flex circuitswith a chromium seed layer as opposed to Ti/W seed layer. It can beobserved that flex circuit with 10 nm of chromium did reach the presettemperature required for gold/gold bonding. Therefore, chromium seedlayer has higher absorption characteristics compared to Ti/W seed layerfor a YAG laser.

[0070]FIG. 16 illustrates temperature increase versus time in a 3-layertape with a 20 nm chromium layer, a tape with a 5 nm chromium layer, anda tape with no chromium layer. As can be seen in FIG. 16, only the flexcircuit with a 20 nm chromium layer indicated a rapid temperature rise.

[0071] Since it was established that chromium thickness is essential tothe integrity of the gold/gold laser bond, when a YAG laser is used, anoptimum chromium thickness was selected as a base line. Referring toFIG. 13, a chromium thickness over 15 nm does not decrease transmissiondrastically. Based on FIG. 15, a chromium thickness of manometers is theabsolute minimum required thickness to provide a successful laser bond.FIG. 15 also illustrates that a flex circuit with 15 nm of chromiumexhibit a much faster temperature rise in the copper trace, resulting inless or no damage to the Kapton tape. Therefore, 15 nm of chromium isoptimum to provide a reliable and repeatable laser bond.

[0072] Some chromium diffusion into the copper is expected during thesubsequent sputtering of chromium as a seed layer and plating processesduring manufacture of the flex circuits. Diffusion of the chromium intothe copper is a time and temperature dependent process, and it isdifficult to determine the amount of chromium that will be diffused intothe copper during these processes. Normally, it is estimated thatmaximum amount of diffused chromium is under 5 nm. Based on thesefactors, a minimum chromium thickness after the sputtering process wasestablished as 20 nm. This thickness should gurantee a minimum chromiumthickness of 15 nm after the completed manufacture of the flex circuit.

[0073]FIG. 17 shows the results of a laser bonding experiment toevaluate the laser bondability of a flex circuit having about 5 nm ofchromium as a seed layer. In this experiment the bond force was variedfrom 20 to 140 grams (20, 40, 60, 80, 100 and 140 grams), and the laserpulse length was varied from 2 to 40 milliseconds (2, 7, 10, 20, 30 and40 milliseconds). The fixed factors in this experiment are die nesttemperature, laser current, maximum feed back temperature andtemperature rise time. By varying the laser energy no bond was formedbetween the TAB lead 208 and the die pad 210. This is due to low laserenergy absorption of the flex circuit due to insufficient thickness ofchromium seed layer.

[0074] Table I indicates the test conditions and test results forseveral experiments. These tests covered a large cross-section ofoperating conditions, covering from no visible effect on the bond tofull Kapton damage. Based on the results illustrated in Table I, it wasconcluded that the existing YAG laser is not capable of bonding existingflex circuits with low chromium thickness.

[0075] A 3-layer flex circuit with 20 nm of chromium with an adhesivelayer between the Kapton, and copper trace was tested. A successfulgold/gold laser bonding was achieved with a laser power set at 10 W,pulse length set at 20 ms, bond force set at 140 grams, and die nesttemperature set at 100 degrees C. No mechanical damage was observed inthe die pad area. This is an indication that neither the laser energy orthe force caused any mechanical damage to the die pad area.

[0076] Table II indicates the test conditions and test results for sevenexperiments. For grading the laser bond results; an “X” quality bond isdefined as a bond that has a cross section similar to the thermalcompression bonded die, with the same or better peel strength. A “B”quality bond is a bond that still has an acceptable bond strength, butthe Kapton joint has been degraded due to higher temperatures (a “B”quality bond may still be acceptable). A “C” quality bond is when thebond is formed, but the bond strength is lower than that of thermalcompression bonded parts. A “F” quality bond is defined as a situationthat a bond was not formed between the copper trace, and the die pad (inmost cases Kapton burned due to increased localized temperature).

[0077] By increasing the pulse length from 5 to 10 milliseconds in TestNo. 2, bond quality improved drastically, but in this case Kapton didburn in one die site. By reducing the pulse length again to 5milliseconds, and increasing the laser power (by means of increasing thelaser current), the bonds became weak again, but burned Kapton was notobserved any more. To further improve the bonding, the laser power wasincreased a second time by increasing the current. In Test No. 4, good,clean bonds were formed and no damage to the Kapton was observed. A peeltest of parts built with these set of parameters indicated a good peelstrength also. Joint strength was further improved by increasing thelaser power. In Test No. 5 the power was increased by increasing thepulse length from 5 to 10 milliseconds. In this case the joint strengthimproved drastically, but some burned Kapton was also observed. In thecase of Tests No. 2 and 5 the burned Kapton was on the copper lead side,and there were no openings exposing the copper lead. Therefore, it issuspected that the adhesive layer between the Kapton and copper lead hasburned. In Test No. 6, the laser current was maintained at 19 amps, butpulse length was increased from 10 to 15 milliseconds. This resulted ina laser over energy which burned several holes all the way through theKapton, without causing any connection between the TAB lead and the diepad.

[0078] Test No. 7 is a repeat of the Test No. 5, with a smaller probeforce. In Test No. 7 probe force was reduced from 140 grams to 100grams. In this case, very much similar to Test No. five, excellent bondswere observed, with high joint strength. However, a possible tape damagewas observed in one die site. In this case also, there was no exposedcopper trace or TAB lead. TABLE I Force Laser Current Pulse Length MaxTemp. Item Grams Amp milli-sec Setting Observation 1 140 17 5 0.4 Nobond/No damage to flex 2 140 17 30 0.4 No bond/No damage to flex 3 14019 5 0.4 No bond/No damage to flex 4 140 19 30 0.4 No bond/No damage toflex 5 100 17 5 0.4 No bond/No damage to flex 6 100 17 30 0.4 No bond/Nodamage to flex 7 100 19 5 0.4 No bond/No damage to flex 8 100 19 30 0.4No bond/No damage to flex 9 140 19 30 0.6 No bond/Flex started to burn10 140 19 30 0.8 No bond/Some flex damage observed 11 140 19 30 1 Nobond/Flex damage clearly observed 12 140 19 50 1 No bond/Some flexdamage observed 13 140 19 50 2 No bond/Flex damage clearly observed 14140 19 30 5 No bond/Flex damage clearly observed 15 140 19 30 9 Nobond/Excessive flex damage 16 140 19 50 9 No bond/Excessive flex damage

[0079] TABLE II Force Laser Pulse Max Temp. Bond Test Grams Ampmilli-sec Setting Quality FIG. Observation 1 140 17 5 0.6 C 18 Weak bondformed at most bond sites 2 140 17 10 0.6 B 19 Acceptable bond formed,but burned kapton in one site 3 140 17.5 5 0.8 C 20 Weak bond formed atmost bond sites 4 140 19 5 0.8 A 21 Good bond formed, no damage tokapton 5 140 19 10 0.8 B 22 Excellent bond formed, but kapton burned insome sites 6 140 19 15 0.8 F 23 Burned kapton, no bonds were formed 7100 19 10 0.8 B 24 Excellent bond formed, but kapton burned in one site

[0080] Based on the results stated in Table II, a bondability window for3-layer tape may be defined as follows: Bond Force: 100-140 grams LaserCurrent:  17-20 Amps Pulse Length:  5-10 milliseconds Maximum SetTemperature:  0.6-0.8

[0081] Experiments were also performed utilizing a 2-layer tape with 20manometers of sputtered chromium. An experimental design was set-up toevaluate effects of force, pulse length, and laser power on the qualityof the bond. This experiment was set-up with the variables force, pulselength, and laser power tested at three levels, resulting in 27individual tests and 27 bonded parts utilizing the FPC laser. All 27parts passed visual inspection, indicating no damage to Kapton orbarrier. The Kapton was then etched to expose the TAB lead. A shear testand a pull test were performed on the 27 parts to evaluate the bondstrength. The shear and pull tests indicated a bond strength of wellover 200 grams for higher laser powers. Table III indicates the testconditions and the bond strength results for the 27 experiments. TABLEIII Bond Laser Shear Test Force Bond Time Power Strength Push StrengthNumber Grams Milliseconds Watts Grams Grams 1 310 20 5.0 0 0 2 310 206.2 82 106 3 310 20 8.5 176 177 4 310 40 5.0 0 0 5 310 40 6.2 90 137 6310 40 8.5 182 169 7 310 60 5.0 0 0 8 310 60 6.2 131 132 9 310 60 8.5186 191 10 360 20 5.0 0 0 11 360 20 6.2 139 112 12 360 20 8.5 189 165 13360 40 5.0 0 0 14 360 40 6.2 146 154 15 360 40 8.5 205 201 16 360 60 5.00 0 17 360 60 6.2 105 177 18 360 60 8.5 225 224 19 412 20 5.0 0 0 20 41220 6.2 88 165 21 412 20 8.5 211 207 22 412 40 5.0 0 0 23 412 40 6.2 178198 24 412 40 8.5 222 195 25 412 60 5.0 0 0 26 412 60 6.2 148 177 27 41260 8.5 210 193

[0082] The experiments established that gold to gold windowless TABbonding is feasible. Shear strengths of well over 200 grams can beachieved easily and repeatedly. No Kapton or barrier damage was observeddue to the laser bonding process. Based on the results stated in TableIII, a bondability window for 2-layer tape may be defined as follows:Low Medium High Bond Force 310 grams 360 grams 420 grams Pulse Time 20msec 40 msec 60 msec Laser Power 5 watts 6.2 watts 8.5 watts

[0083] The present invention eliminates the need for the TAB window andthe associated encapsulation of the prior art and results in a planarTAB connect process. This in turn results in lower cost, higherreliability and ease of serviceability.

[0084] The tape 104 is then stepped to a heat and pressure station 122.As previously discussed with respect to FIGS. 9 and 10, an adhesivelayer 84 exists on the top surface of the barrier layer 30 formed on thesilicon substrate. After the above-described bonding step, the silicondies 120 are then pressed down against the tape 104, and heat is appliedto cure the adhesive layer 84 and physically bond the dies 120 to thetape 104.

[0085] Thereafter the tape 104 steps and is optionally taken up on thetake-up reel 124. The tape 104 may then later be cut to separate theindividual TAB head assemblies from one another.

[0086] The resulting TAB head assembly is then positioned on the printcartridge 10, and the previously described adhesive seal 90 is formed tofirmly secure the nozzle member to the print cartridge, provide anink-proof seal around the substrate between the nozzle member and theink reservoir, and encapsulate the traces in the vicinity of theheadland so as to isolate the traces from the ink.

[0087] Peripheral points on the flexible TAB head assembly are thensecured to the plastic print cartridge 10 by a conventional melt-throughtype bonding process to cause the polymer flexible circuit 18 to remainrelatively flush with the surface of the print cartridge 10, as shown inFIG. 1.

[0088] The foregoing has described the principles, preferred embodimentsand modes of operation of the present invention. However, the inventionshould not be construed as being limited to the particular embodimentsdiscussed. As an example, while the present invention was described inconjunction with the bonding of conductive traces on a TAB circuit tothe silicon substrate of an inkjet printhead, the present method andapparatus for the solderless electrical connection of two contactelements by using a laser light beam attached to a fiber optic system isapplicable to bonding other types of electrical members to each other.Likewise, while the present invention was described in conjunction withsolderless gold to gold bonding of electrical members to each other, thepresent method could be used for the solderless bonding of otherconductive metals. Thus, the above-described embodiments should beregarded as illustrative rather than restrictive, and it should beappreciated that variations may be made in those embodiments by workersskilled in the art without departing from the scope of the presentinvention as defined by the following claims.

What is claimed is:
 1. A method of bonding electrical leads of a TABcircuit to electrical contact bumps by a laser beam having a specifiedwavelength, comprising the steps of: providing the electrical leads witha seed metal on the top surface the electrical leads, said seed metalhaving a suitable absorption at the wavelength of the laser beam;aligning the electrical leads with the electrical contact bumps with theelectrical leads above the electrical contact bumps; holding theelectrical leads and electrical contact bumps in contact at a bondsurface with an optical fiber; and bonding the electrical leads andelectrical contact bumps at the bond surface by directing the laser beamthrough the optical fiber.